1. Field of the Invention
This invention relates to a density correction method applied to image formation in a heat developing apparatus in which a photothermographic material including a heat-developable light-sensitive material and a light- anf heat-sensitive recording material is exposed to laser light, etc. The present invention also relates to a heat developing apparatus capable of density correction according to the method.
2. Description of the Related Art
Reduction of waste liquid in the medical field has been keenly demanded for environmental conservation and space saving. Too meet the demand, it has been desired to establish techniques regarding heat-developable light-sensitive photographic materials for diagnosis and for photographic applications which can be efficiently exposed with a laser image setter or a laser imager to form a crisp black- anf-white image with high resolution and sharpness. Such heat-developable light-sensitive photographic materials will provide customers with a simpler and more environmentally friendly heat development system involving no wet chemical processing.
While there has been the same demand in the field of general image formation, images for diagnosis characteristically demand high quality in sharpness and graininess for precision and a cool black tone for facilitating diagnosis. From this viewpoint, hard copy systems using pigments or dyes which are currently available as general image forming systems, such as ink jet printers and electrophotographic apparatus, are not satisfactory as an output system for medical diagnostic imaging modalities.
Under these circumstances, a dry system recording apparatus involving no wet processing has recently engaged attention. Light- and heat-sensitive materials or heat-developable light-sensitive photographic films (hereinafter inclusively referred to as photothermographic recording materials) are used in such a recording apparatus. In the dry system recording apparatus, a photothermographic recording material is irradiated (scanned) with laser light to form a latent image in an image exposure section, brought into contact with a heating unit to perform heat development in a heat development section, and, after cooled, discharged out of the apparatus. The dry system is able to eliminate the waste liquid disposal problem associated with a wet system.
Thermal image formation systems using organic silver salts are described, e.g., in U.S. Pat. Nos. 3,152,904 and 3,457,075 and xe2x80x9cThermally Processed Silver Systemsxe2x80x9d by B. Shely in J. Sturge, V. Walworth and A. Shepp (eds.), Imaging Processes and Materials Noblette""s Eighth Edition, 1996, 2. Photothermographic recording materials generally have a light-sensitive layer comprising a catalytic amount of a photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver salt) and, if needed, a toning agent for controlling the tone of developed silver, all dispersed in a binder matrix. After imagewise exposure, the photothermographic recording material is heated to an elevated temperature (e.g., 80xc2x0 C. or higher) to induce redox reaction between the silver halide or the reducible silver salt (acting as an oxidizing agent) and the reducing agent thereby to form a black silver image. The redox reaction is accelerated by the catalytic activity of the latent image of silver halide generated on exposure. Therefore, the black silver image is formed in the exposed area. Photothermographic recording materials and systems based on this principle have been disclosed in many documents, such as U.S. Pat. No. 2,910,377 and JP-B-43-4924. Commercially available medical imaging systems using a photothermographic recording material include Fuji Medical Dry Imager FM-DP L supplied by Fuji Medical Systems Inc.
Photothermographic recording materials using an organic silver salt are produced by coating a support with an organic solvent-based coating composition or an aqueous dispersion containing polymer particles as a main binder. Excluding the necessity of extra steps such as solvent recovery, the method using the aqueous dispersion is advantageous for equipment simplicity and suitability to large volume production.
In the above-described heat development systems, image data from an image data supply source are subjected to various image processings, such as sharpness processing and shading correction, to obtain data suited to an image recording method, and an image is recorded on a recording material according to the processed data. The apparatus for image recording is required to always output an image of prescribed density according to the image data sent from various image data supply sources, such as a diagnostic imaging modality and an image reading unit. For example, in an image forming apparatus which receives 10-bit digital data and forms an image according to the data, where 300 digital data correspond to a density of 1.2, the apparatus should always output an image of density 1.2 for 300 digital data.
However, there are variations among individual heat developing apparatus, and the recorded image density also varies depending on the environment in which the apparatus is installed. It is impossible for all the apparatus to output an image of a prescribed density based on a piece of image data supplied. Therefore, density correcting conditions are programmed into a general image recording apparatus so as to output an image having a prescribed density according to image data, whereby the image data are corrected (so-called calibration). Further, because an image recording apparatus changes its condition with the progress of recording or with time, it is difficult to obtain an image of a prescribed density in a stable manner for a long period of time. A laser recording apparatus, for instance, not only suffers from contamination, wear or like changes in its optical system in long-term use but is exposed to environmental changes, particularly in temperature. As a result, the density of the output image based on the same data varies with time. It is therefore necessary to periodically reset the density correcting conditions.
The density correcting conditions are set or reset as follows. A chart for density correction having patches of various densities is outputted from a heat developing apparatus. For example, a density correcting chart 500 shown in FIG. 5 is used. The chart 500 has 24 monochromatic patches of densities varying from 0 to 23. The density of each patch of the output is measured with a built-in densitometer 600 shown in FIG. 6. The built-in densitometer 600 basically comprises a light emitter 601 and a light sensor 602.
In the example shown in FIG. 6, a red LED is used as the emitter 601, and the sensor 602 is adapted to detect transmitted light. The sensor 602 detects light quantity of transmitted light 500a through the chart 500 (output) by the photo receptor 602a, converts the transmitted light quantity into electrical signals, and outputs the signals to a recording controller 37 (see FIG. 2).
In the built-in densitometer 600, the chart 500 is irradiated with a predetermined amount of light 601a. The sensor 602 receives light 500a transmitted through the chart 500 at the receptor 602a and outputs signals corresponding to the amount of detected transmitted light to the recording controller 37 (FIG. 2). The recording controller 37 processes the signals into densities, compares the densities (measured with the built-in densitometer 600) with the densities of the images which are to be recorded by the heat developing apparatus (i.e., the image densities corresponding to image data) to draw a calibration curve.
The problem is that built-in densitometers generally employed are designed to measure densities of wet-processed images formed by silver salt color formation. It follows that they fail to stably execute accurate measurements when applied to images formed by dye color formation as in thermo-recording. In detail, commonly used built-in densitometers generally have different sensitivities depending on wavelength. When used to measure the density of wet-processed images by silver salt color formation, the densitometer stably gives measurements in good agreement with visual densities because the developed color shows constant absorption irrespective of wavelength. Conversely, because the images by dye color formation show different absorptions with wavelength, the results from the built-in densitometer differ from the visual densities. For example, the densities of a wet-processed image by silver salt color formation and a dry-processed image by dye color formation, both having a visual density of 2.0, are 2.0 and 1.8, respectively, as measured with the same built-in densitometer. Like this, the measured density of dry-processed dye color images tends to be lower than the visual density. Moreover, the measurement results vary with the individual built-in densitometer.
For this reason, density correcting conditions which are set based on the chart density data as measured with a built-in densitometer are not accurate in performing imaging on a recording material relying on dye color formation. Such inaccurate setting fails to provide high quality images properly corresponding to the image data fed from an image data supply source. For example, the resulting recorded images are apt to have a higher density than programmed as a whole. The above-mentioned various problems occur in image recording using various recording materials for a dry system as well as the recording materials relying on dye color formation.
JP-A-9-307767 (corresponding to U.S. Pat. No. 6,072,513) illustrates a previously-proposed method of density correction for settling these problems. This method enables setting proper density correcting conditions for a dry system image recording apparatus irrespective of differences among different built-in densitometers, whereby high quality images can be recorded in agreement with image data fed from an image data supply source. According to the method, the details of which will be described later, a built-in densitometer is calibrated for density correction by use of a reference chart having known densities corresponding to visual densities.
The method is based on the premise that a recording material has an unchanged color tone. Strictly speaking, however, the tone of a recording material undergoes change with time as hereinafter explained.
A photothermographic recording material used in a heat developing apparatus (sometimes referred simply to xe2x80x9crecording materialxe2x80x9d), of which the details will be described later, shows instable and considerable scatter in red transmitted density in a high density area. The state-of-the-art built-in densitometers have very poor accuracy when used to measure densities in a high density area of this type of recording materials.
Further, since the tone change of the recording material is caused by change of the recording material formulation, change of the recording material itself with time, and the like, a change in tone of the recording material leads to an increased error of measurement with the built-in densitometer, resulting in a greater difference from the visual density. It has turned out that the difference from the visual density is conspicuous in a high density area. This is considered to be because, in a densitometer using a monochromatic light source (e.g., an LED or a laser), the quantity of light transmitted through a higher density area and detected by the receptor of the densitometer is smaller, which will lead to an exaggerated error of measurement. The resultant error of measurement gives rise to a serious problem particularly in diagnosis using low-to-high density areas, such as mammography.
The present invention provides, in its first aspect, a method of density correction in a heat developing apparatus in which a photothermographic recording material inclusive of a heat-developable light-sensitive material and a light- and heat-sensitive recording material is thermally developed by light or heat application,
the method comprising having the heat developing apparatus output a density correcting chart for setting density correcting conditions, measuring the image densities of the density correcting chart with a densitometer built in the heat developing apparatus using a monochromatic light source, and correcting density correcting conditions for image recording in the heat developing apparatus based on the measured densities, wherein:
the density correcting conditions are corrected by using only the densities of low to middle density areas out of the densities of low to high density areas as measured with the built-in densitometer and corrected based on the density correcting chart.
According to the density correcting method of the invention, because the density data from a high density area are not adopted, there is no difference from the visual density even where a recording material showing large scatter in red transmitted density is used or even where a recording material has undergone a change in tone.
In a preferred embodiment of the first aspect of the invention, densities based on a standard tone curve of the photothermographic recording material are extrapolated into the high density area. According to this embodiment, the date of the high density area are corrected to agree with the visual densities so that density correction can be made to substantially eliminate differences from the visual densities.
In a still preferred embodiment of the first aspect of the invention, the density value of the high density area, which is extrapolated based on the standard tone curve of the photothermographic recording material, is adapted to be corrected by a user. This embodiment provides a user-friendly density correction method, allowing a user to alter the density of the high density area at his or her own discretion.
The present invention also provides, in its second aspect, a heat developing apparatus for thermally developing a photothermographic recording material inclusive of a heat-developable light-sensitive material and a light- and heat-sensitive recording material by light or heat application,
which comprises (1) means for outputting a density correcting chart, (2) a built-in densitometer for measuring the image densities of the density correcting chart using a monochromatic light source, and (3) means for correcting the image density values of the built-in densitometer by comparison with known image densities supplied from the means for outputting a density correcting chart so as to agree with the known image densities, wherein
the means for correcting the image density values uses only the densities of low to middle areas out of the densities of low to high density areas as measured with the built-in densitometer and corrected based on the density correcting chart.
According to the second aspect of the invention, differences from the visual density do not occur even in using a recording material showing large scatter in red transmitted density or even where a recording material has undergone a change in tone.
In a preferred embodiment of the second aspect of the invention, the heat developing apparatus further comprises (4) means for extrapolating density values of a high density area based on a standard tone curve of the photothermographic recording material into the high density area and (5) means for altering the density values in the high density area which allows a user to alter density values in the high density area at the discretion of the user. Because a user can add alteration to the density of the high density area at his or her own discretion, the apparatus according to this embodiment is very user-friendly.
In a conceivable embodiment of the method and the apparatus according to the invention, the monochromatic light source of the built-in densitometer is replaced with a white light source, such as a white lamp. Use of multicolor white light overcomes the red transmitted density scatter problem. That is, the measurements with the densitometer are not influenced by the change in tone of the photothermographic recording material.
In this case, it is recommended to insert an optical filter having such transmission characteristics as to bring agreement with vision between the white light source and the receptor of the densitometer. This configuration furnishes density values close to visual densities even where the recording material has undergone change in tone or even where the densitometer involves large measurement errors. The scatter of measurement values can further be lessened by increasing the luminance of the densitometer light source to intensely irradiate the high density area.